It's where this thing is a "flying boat" style that I keep getting the idea your gonna fly it off of water . I guess snow , even fine sand , or of course well cut grass could provide for flights where water ain't involved but with that hull I'd take it to my nearest small lake & have at it .
Anyway , power source wise , since you mentioned around 37" of wing , roughly in and around parkflyer size , I think (using the E flight system of motor rating) a "Park 480" size would be perfect .